Standup GitHub Actions CI

[keithm-xmos]

Include:

• [x] Documentation build
• [ ] Code build
• [ ] Tests
  • [x] building
  • [ ] unit (run in GH Actions using xsim)
  • [ ] signal (run on HW at first)

[keithm-xmos]

Currently blocked because demos and tests will not build outside the context of the SDK. Waiting for resolution.

[keithm-xmos]

Resolved by the creation of https://github.com/xmos/fwk_io